Problem: Which of these numbers is prime? ${44,\ 53,\ 65,\ 81,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 44 are 1, 2, 4, 11, 22, and 44. The factors of 53 are 1 and 53. The factors of 65 are 1, 5, 13, and 65. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 53 is a prime number.